1. Field of the Invention
The present invention relates to a composite integrated circuit device having test mode function.
2. Description of the Related Art
For testing a composite integrated circuit (IC) device, normally, it has been necessary to additionally arrange at least one exclusive terminal for inputting test mode setting data (referred as test terminal) to the IC device in addition to normal input terminals and output terminals, or normal input/output terminals. Such normal input terminals and output terminals, or normal input/output terminals are served to receive and/or feed out signals during normal operating condition of the IC device.
The test terminal is used only for receiving test mode setting data. The test modes are executed in response to the inputted test mode setting data by separating function blocks, for example central processing unit (CPU), memories, logic circuits, and the like, built in the composite IC device from each other. In other words, the conventional composite IC device has at least one additional exclusive test terminal never used for normal operation of the IC device.
Such IC device provided with at least one additional exclusive terminal for receiving test mode setting data has an advantage that it is possible to freely determine the constitution of a signal and a circuit for setting each test mode so far as how to properly couple each of the function blocks for the normal operation is considered. However, since the composite IC device becomes more complicated and functional, a greater number of test modes are required causing the number of additional exclusive terminals for inputting test mode setting data to greatly increase.
Recently, the composite IC device, referred to as a CPU core formed by modifying an existing CPU, is likely to be kept more functional and complicated in a system-on-chip manner. So, such IC device keeps more terminals accordingly.
In order to cope with such the tendency, following disadvantages have to be overcome.
(1) The net manufacturing cost is increased as the terminals become more in number.
(2) Since the terminals dedicated for the normal opera&ion become more in number, the area of a package restricts an upper limit number of test terminals.
(3) By studying the combination of "0" and "1" inputted into the test terminals for inputting test mode setting data, the content (program or data) stored in the memories of the composite IC device may be easily decoded by a third person. This is problematic in light of protecting software.